This invention relates generally to communication systems and more particularly to radio transceivers used within such communication systems.
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), wireless application protocols (WAP), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and share information over that channel. For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver receives RF signals, removes the RF carrier frequency from the RF signals via one or more intermediate frequency stages, and demodulates the signals in accordance with a particular wireless communication standard to recapture the transmitted data. The transmitter converts data into RF signals by modulating the data in accordance with the particular wireless communication standard and adds an RF carrier to the modulated data in one or more intermediate frequency stages to produce the RF signals.
As the demand for enhanced performance (e.g., reduced interference and/or noise, improved quality of service, compliance with multiple standards, increased broadband applications, et cetera), smaller sizes, lower power consumption, and reduced costs increases, wireless communication device engineers are faced with a very difficult design challenge to develop such a wireless communication device. Typically, an engineer is forced to compromise one or more of these demands to adequately meet the others. For instance, an engineer may choose a direct conversion topology (i.e., convert directly from an RF signal to a base-band signal or directly from a base-band signal to an RF signal) to meet size requirements and/or broadband application requirements. However, for direct conversion transceivers, noise and/or interference increases due to local oscillation leakage, non-linearities due to component mismatches and/or process variations are more detrimental to overall performance, and DC offsets, which result from a slight offset between the transmitting frequency of one wireless communication device and the frequency of the receiver in another wireless communication device, are more pronounced.
As is known, local oscillation leakage results from imperfections of the mixers within a transmitter that allow the local oscillation, which equals the RF, to be present in the resultant RF signal. The local oscillation leakage can be minimized by using multiple IF stages within the transmitter. In such an implementation, each IF stage uses a local oscillation that has a significantly different frequency than the RF, with the sum of the multiple local oscillations equals the RF. Since each local oscillation has a significantly different frequency than the RF, each local oscillation is outside the RF band of interest (i.e., the frequency spectrum of the resulting RF signal). But this requires an abandoning of the direct conversion topology and its benefits with respect to size reduction, power consumption reduction, reduced costs, and reduced complexity for broadband applications.
Costs of manufacturing a radio frequency integrated circuit (IC) may be reduced by switching from one integrated circuit manufacturing process to another. For example, a CMOS process may be used instead of a bi-CMOS process since it is a more cost affective method of IC manufacture, but the CMOS process increases component mismatches, increases temperature related variations, and increases process variations. As such, noise, local oscillator leakage, non-linearities and other factors that negatively impact an RF IC performance are increased for a CMOS process. Thus, in many RF IC applications, a designer chooses between cost savings and performance.
As is further known, many wireless communication standards provide for varying the transmitting power of the transmitter based on received signal strength of the wireless communication device receiving the transmission to conserve power. For instance, if the received RF signal is very strong, the receiver can easily recapture the embedded data. In such an instance, the transmission power level of the transmitter can be reduced and still provide a sufficiently strong RF signal to enable the receiver accurately recover the embedded data. Conversely, when the received signal is too weak, the receiver cannot accurately recover the embedded data, thus the transmission power level needs to be increased. Typically, the transmitter power is increased by increasing the gain of its power amplifier. The gain of the power amplifier is increased by changing the bias level of the input signal. While this increases the gain, the linearity of the power amplifier is adversely affected, which adversely affects the performance of the transmitter and the overall radio.
Therefore, a need exists for a low power, reduced size, reduced cost, and enhanced performance radio, radio transmitter, radio receiver, and/or components thereof.
These needs and others are substantially met by the programmable mixer disclosed herein. A programmable mixer includes a 1st mixing stage, a 2nd mixing stage, a coupling element, and a compensation module. The 1st mixing stage is operably coupled to mix one leg of a differential input signal with a differential local oscillation. The 2nd mixing stage is operably coupled to mix the other leg of the differential input with the differential local oscillation. The coupling element, which may be a resistor, inductor, MOSFET, or other such components, couples the 1st and 2nd mixing stages together.
The compensation module is operably coupled to the 1st mixing stage and/or the 2nd mixing stage to modify the operational characteristics (e.g., current, impedance, gain, et cetera) of the 1st and/or 2nd mixing stages based on a control signal. By adjusting the operational characteristics of the 1st and/or 2nd mixing stages, the compensation module is attempting to more closely match the operational characteristics of the 1st mixing stage with those of the 2nd mixing stage. For example, the compensation module may adjust current flow in the 1st and/or 2nd mixing module such that the current flow through the 1st mixing module closely matches the current flow through the 2nd mixing module, which reduces imbalances and correspondingly reduces local oscillation leakage.
The programmable mixer may be used in an up-conversion module of the transmitter and/or in the down-conversion module of the receiver. Whether the programmable mixer is used in the transmitter path and/or receiver path, the compensation module may be constructed in a variety of embodiments. For instance, the compensation module may include single current source operably coupled to the 1st mixing source and another current source coupled to the 2nd mixing source to provide an offset current for the 1st and 2nd mixing stage. In an alternative embodiment, the compensation module may include a plurality of current sources to compensate the 1st mixing stage and a 2nd plurality of current sources to compensate the 2nd mixing stage. Alternatively, the compensation module may include current sources that subtract current from the 1st mixing stage and/or 2nd mixing stage.